Atomic mass:
A property closely related to an atom’s mass number is its atomic mass. The atomic mass of a single atom is simply its total mass and is typically expressed in atomic mass units or amu. By definition, an atom of carbon with six neutrons, carbon-12, has an atomic mass of 12 amu.
Relative atomic mass:
Since an element’s isotopes have different atomic masses, scientists may also determine the relative atomic mass—sometimes called the atomic weight—for an element. The relative atomic mass is an average of the atomic masses of all the different isotopes in a sample, with each isotope's contribution to the average determined by how big a fraction of the sample it makes up.
Isotopic mass:
Isotopic mass refers to the average mass of all the isotopes of a specific element. An element can contain a defined number of protons in their nucleus, but the number of neutrons can vary. This can give us elements with the same atomic number but different atomic mass.
